Electronic devices often need to perform the function of a switch, providing either a very high or a very low resistance between two pins or other contact terminals on the outside of the package of the device. A control circuit switches the switch on and off, so as to provide either the very high or very low resistance between the pins (the short circuit is not a matter of glitches: the switch provides a short-circuit persistently if switched-on). Devices with such switches are applied for example in DC/DC converters and other circuits wherein the power supply of other circuits has to flow through the switch.
Usually, this the switchable low resistance is realized by incorporating the main current channel of a switching transistor between the two pins. If the resistance has to be very low it has been found that it becomes difficult and/or expensive to test whether the transistor performs as required. Normally, such a test is performed by connecting the pins to a resistance probe and measuring the resistance connected to the probe, the device being rejected if the measured resistance is too high. However, if the resistance between pins has to be very low, contact resistance of the contact between the probe and the pins may become an uncertain factor in the measurement. It is expensive to ensure that the resistance is low and predictable, as this may slow down testing and/or require expensive probes that need constant maintenance, especially if very small packages have to be tested.
One way of overcoming this problem is to use a four-point measurement. In a four-point measurement current is applied to the device under test via a first pair of connections and the resulting voltage across the device is measured via a second pair of connections. Thus, the measured voltage drop does not include the voltage drop over the contact resistance to the first pair of connections, via which the current is applied. However, this test is also expensive, in that it requires two additional connections to the package of the device if the test is performed after packaging.
Under some circumstances, the device may have other pins that can be temporarily used for the purpose of a four-point measurement during testing. An example of this can be found in European patent application No. 720 023. However, this technique is not generally applicable, as it depends on the type of circuit being used.
Amongst others, it is an object of the invention to provide for a test technique and a device designed for such a test technique for testing the resistance of a switching transistor with only two contact terminals to the transistor, while avoiding unreliable results due to an unpredictable contact resistance.
The method according to the invention is described in claim 1. According to the invention, at least two switching transistors are connected in parallel between the contact terminals. Three resistance values are measured with a probe connected to the contact terminals: one with both transistors switched on, and two with one transistor switched on and the other transistor switched off. From the three measurements, three resistances can be computed: the resistances of the parallel branches inside the package (each of which contains a switching transistor) and the resistance of the connection between the probe and the parallel branches. Thus, the contact resistance can be eliminated from the measurement of the resistance of the switching transistors.
Preferably, the different switching transistors are incorporated in a single semi-conductor substrate, the main current channel of each transistor being connected to a different bondpad, the bondpads being connected to the same contact terminal via different bonding wires. This reduces the effect of electromigration and allows incorporation of both the resistance of the switching transistor and the bonding wire in the measured resistance from which the contact resistance is eliminated.
In an embodiment, the bondpads for the different switching transistors are mutually connected inside the integrated circuit by a relatively high resistance (compared with the xe2x80x9conxe2x80x9d resistance of the switching transistors). This allows a four-point measurement of the switching resistance of each switching transistor is before the device is finally packaged. At this stage more contacts are possible than just the two contact terminals. Hence, one can use one bondpad of one transistor to supply the current and the bondpad of the other transistor to measure the resulting voltage drop.